Controlling the rheology of a paint, both in the stage of its manufacture, and during its transport, storage or use, remains a priority at the current time. The wide variety of constraints observed in each of these steps relates to a multiplicity of different rheological properties. Nevertheless, it is possible to summarise the requirement of the skilled man in the art in obtaining an effect of the thickening of the said paint, both for reasons of stability over time, and for a possible application to a vertical surface, lack of spattering during use, or of sagging after application, etc. As a consequence, the products which contribute to this regulation of the rheological properties have been designated by the term “thickening agents”.
Historically, since the 1950s cellulose-based gums and products have been used, one of the essential characteristics of which is their high molecular weight. However, these compounds have a number of disadvantages, such as their instability over time (see document U.S. Pat. No. 4,673,518), the need to use a large quantity of them (see document EP 0 250 943 A1), and their production costs, notably in terms of waste treatment (see document U.S. Pat. No. 4,384,096).
Thickening agents called “associative” thickening agents were then created: these are water-soluble polymers having insoluble hydrophobic groups. Such macromolecules have an associating character: when introduced into water, the hydrophobic groups tend to assemble in the form of micellar aggregates. These aggregates are linked together by the hydrophilic parts of the polymers: a three-dimensional network is then formed which causes the viscosity of the medium to be increased. The operating mechanism and their characteristics are now well known and described, for example in the documents “Rheology modifiers for water-borne paints” (Surface Coatings Australia, 1985, pp. 6-10) and “Rheological modifiers for water-based paints: the most flexible tools for your formulations” (Eurocoat 97, UATCM, vol. 1, pp 423-442).
Among these thickening agents, a distinction is made between the category of HEURs (Hydrophobically modified Ethylene oxide URethane), and HASEs (Hydrophobically modified Alkali-Soluble Emulsions). The first designate polymers resulting from the synthesis between a compound of the polyalkylene glycol type, a polyisocyanate, and an alkyl and/or aryl associative monomer consisting of a hydrophobic terminal group. The second designate polymers of (meth)acrylic acid, of an ester of these acids and of an associative monomer having a polymerizable group and an oxyalkylated chain terminated by a hydrophobic group.
HEURs can give water-based paints many properties and advantages, notably depending on the nature of their associative monomer. The following patent applications registered by the company COATEX may be cited: EP 0 639 595 A1, which proposes hydrophobic groups having 4 to 36 carbon atoms to increase the Brookfield™ viscosity, WO 02/102868 A1, which describes the use of plurystyrylphenols with more than 40 atoms to increase the viscosity, whatever the shearing gradient, and finally EP 1 425 325 A1, which reveals an associative monomer consisting of di- and tristyrylphenol, enabling an excellent pigmentary compatibility and a high viscosity with low and medium shearing gradient to be obtained.
HEURs are nonetheless still chemical species which are not very water-soluble, and which require solvents or surfactants for solutioning, when their active matter % exceeds approximately 15%. This problem is explained in document EP 0 682 094 A1: the proposed solution is based trivially on the use of surfactants. In addition, it will be noted that thickening agents of the HEUR type, which are highly concentrated and contain surfactants, had been sold by the company COATEX™ under the names COAPUR™ 5035 and COAPUR™ 6050, before the priority date of this application, and from 1993 in the case of the first of these.
And the use of solvents and surfactants to formulate the HEUR-type thickening agent poses a number of problems. Firstly it will be recalled that solvents are subject to increasingly draconian legislation aimed at restricting and even prohibiting their use in paints. As for the surfactants of the thickening formulation, their nature is such that they sometimes have a problematic impact on paints, through interaction with the other surfactants contained in these paints. Consequently, formulating HEURs only in water is only possible provided the polyurethane concentration is restricted (of the order of 20% by weight), which makes these thickening agents ineffective in their untreated state, or provided their intrinsic thickening efficiency is reduced, which is not desirable.
The skilled man in the art therefore considers HASE-type thickening agents. Where the latter are concerned, the choice of the hydrophobic group of the associative monomer determines the varied rheological properties. The following patent applications filed by COATEX™ may be cited with this regard: EP 0 577 526 A1, which describes a fatty chain with linear or branched units of the alkyl and/or aryl type, having 26 to 30 carbon atoms, to develop high viscosities under a low shearing gradient, and EP 1 778 797 A1, which describes a branched terminal chain comprising 10 to 24 carbon atoms, to improve the pigmentary compatibility, and increase the viscosity generally.
Among the most efficient HASE thickening agents, in terms of thickening power, are the emulsions containing grafted alkyl phenols, which play the role of hydrophobic terminal chain. As examples, the products Rheotech™ 2100 and Rheotech™ 3000 sold by the company COATEX™ may be cited. However, alkyl phenols are currently widely suspected of being carcinogenic, and dangerous for reproduction; although still tolerated in the paints industry they nonetheless remain in the firing line of the legislative institutions in this area, notably the European ones. These products do not therefore constitute a satisfactory response for the skilled man in the art, even when they are grafted on to a polymer of high molecular weight.